1. Field of the Invention
This invention relates to an alkaline storage battery, typically represented by a nickel-hydrogen storage battery and nickel-cadmium storage battery, which has an enhanced capacity and to a process for producing the same.
2. Description of Related Art
In recent years, with the rapid progress of widespread use of portable and/or cordless instruments, a demand is increasing for secondary batteries which are small in size, light in weight and have a high energy density to serve as the power source of such instruments. On the market, secondary batteries which have a particularly high capacity and are inexpensive are awaited. Accordingly, capacity increase, cost reduction and improvement of reliability on the market are eagerly awaited for alkaline storage batteries represented by nickel-hydrogen storage batteries and nickel-cadmium storage batteries.
Such alkaline storage batteries have previously been constructed by housing in a battery case an electrode assembly constructed such that a positive electrode plate comprising nickel hydroxide as the principal component and a negative electrode plate are placed to oppose each other with an electrically insulating separator placed therebetween, then pouring a predetermined amount of an alkaline electrolyte into the electrode assembly, and thereafter tightly closing the upper part of the battery case with a sealing plate which also serves as the terminal of either the positive or the negative electrode.
The positive electrode plate herein has hitherto been prepared by preparing an active material paste by kneading an active material comprising nickel hydroxide as the principal component together with water and a water-soluble bonding agent, filling the active material paste in a metallic porous body comprising nickel, followed by drying, and then pressing the resulting electrode to make the thickness uniform and at the same time to increase the packing density of the active material. This process has been disclosed, for example, in JP-A-9-106814.
To secure a necessary amount of positive electrode active material for attaining an increased capacity, the positive electrode plate has been prepared by using a metallic porous body comprising nickel having an increased thickness.
However, when the thickness of the metallic porous body is increased to attain an increased capacity, the degree of difficulty in preparation of the porous body increases, leading to increase in cost. Since the positive electrode is, by nature, of a structural material having a low degree of freedom from deformation by bending, when the thickness of the metallic porous body is increased, the cracking and breaking of the metallic porous body and the peeling and separation of the active material become apt to occur at the positive electrode plate preparation step. Consequently, there arises a possible risk of the active material which has fallen off breaking through the separator present between the positive electrode plate and the negative electrode plate to cause internal short-circuit between the positive electrode and the negative electrode.
Furthermore, since the uniformity of packing of the active material paste decreases when the thickness of the metallic porous body is increased, at the paste where the packing of the active material is dense, the foamed metallic porous body is apt to undergo cracking and breaking at the time when the positive electrode plate is pressed to attain the desired thickness; at the parts where the packing of the active material is sparse, on the other hand, the skeleton of the metallic porous body is apt to be bared. Therefore, there arises a possible danger of the internal short-circuit occurring owing to contact of the positive electrode plate with the negative electrode plate caused by the broken skeleton or bared skeleton breaking through the separator.
Moreover, a thick metallic porous body is difficulty penetrated to the inside by plating, hence the internal skeleton tends to be thin, so that the use of a thick-type metallic porous body is liable to cause lowering of discharge characteristics.